Laser illuminators are a class of light illumination systems in which at least one laser device acts as the source of optical radiation. These illuminator devices are generally more complex and less affordable than their more conventional counterparts that make use of nonlaser optical sources. Laser illuminators then find their best use in niche applications that call for highly-demanding illumination characteristics. For example, combining a laser illuminator to a low-light-level imager, either intensified or not, results in an active night-vision system that enables viewing of scenes in full darkness. Non-emissive objects located at distances that can attain a few kilometers can be illuminated and then reliably detected. Active night-vision systems find their way in both military (visual reconnaissance, surveillance, sniper detection, search-and-rescue operations) and civilian applications (surveillance, evidence gathering, law enforcement, identification of vehicles). The key illumination parameters vary with the intended application, but they relate generally to the radiated laser power, the field of illumination (related to the beam divergence, or angular spreading), which determines the size of the illuminated area at a given distance, and the uniformity of the radiant intensity (power per unit solid angle) within the field of illumination.
As compared to their nonlaser counterparts, several types of laser sources are well suited for use in light illuminators. This is due to some distinctive properties such as their unsurpassed radiance (power per unit area and unit solid angle), the excellent spatial quality of their raw laser output beams, their ability to operate in repetitively-pulsed regime with pulse durations as short as a few nanoseconds, and their nearly monochromatic emission spectrum. This latter property enables efficient rejection of the parasitic background light through the use of a matched narrowband optical filter inserted in front of the low-light-level imager. Laser sources are available with emission wavelengths that now cover a wide portion of the electromagnetic spectrum from the ultraviolet to the mid-infrared. For instance, laser illuminators that radiate near-infrared light invisible to the unaided eye can be built for applications in which covertness is a premium while maintaining an excellent image resolution. Accordingly, the laser emission wavelength can be selected to closely match the peak of the spectral responsivity figure of the low-light-level image sensor, thus providing optimum detection sensitivity. Finally, lasers emitting at wavelengths above 1400 nm (nanometers) out of the retinal hazard region can be integrated in illuminators that are required to be safe in case of inadvertent ocular exposure to the laser radiation.
High-power semiconductor lasers in the form of monolithic linear arrays of several laser diode emitters grown on a common substrate are recognized as the most popular optical sources for use in laser illuminators. These laser devices, generally known as either laser diode arrays or laser diode bars, have a high electrical-to-optical conversion efficiency while their rated lifetimes typically exceed several thousand hours. In addition, their very small size allows them to be packaged in the form of rugged and very compact laser modules. By way of example, FIG. 1 (PRIOR ART) shows a schematic perspective view of a laser diode bar comprising eight individual emitters having their front facets (radiating output apertures) depicted by the hatched rectangular areas. Note, however, that real laser diode bars may comprise from 10 to 80 individual emitters. The laser emitters are equally spaced from each other, and they spread along the X axis, parallel to their semiconductor PN junctions. The width W of each individual emitter along the X axis is typically in the range of 50 μm to 200 μm (micrometers), while the center-to-center spacing S (also referred to as the emitter pitch) is such that the ratio W/S varies from about 20% to 80%. This ratio is often denoted as the fill factor of the laser diode bars. The full width of a laser diode bar is generally set to the 10 mm standard. Nowadays, properly cooled laser diode bars driven with injection currents of several tens of Amperes can radiate tens of Watts of peak laser power.
The hatched rectangular areas shown in FIG. 1 provide a misleading representation of the relative dimensions of the emitter front facets because their extent along the vertical Y axis does not exceed about 1 μm. As a result, the front facet of each laser diode emitter has the shape of a very thin line elongated along the X axis. It is well known that an optical field of wavelength λ escaping from an aperture having one of its transverse dimensions that compares to λ will suffer from sizable optical diffraction effects along this dimension. Depending on its material composition, the emission wavelength of a laser diode bar ranges typically from about 0.8 μm to 1.5 μm. The optical diffraction effects cause the output laser beamlet to diverge appreciably along the vertical Y axis as soon as it escapes from the emitter front facet. As indicated in FIG. 1, the full divergence angle θ⊥ of a laser beamlet typically reaches 80° along the Y axis. Unfortunately, the linear array of highly-divergent beamlets radiated as a whole by a laser diode bar is totally useless for many applications (including illumination) unless it is passed through suitable beam conditioning optics having a high numerical aperture along the Y axis. Optics with high numerical aperture are often qualified as being fast, so that the vertical Y axis in FIG. 1 is generally referred to as the fast axis of a laser diode bar. In contrast, the large emitter width W relative to the emission wavelength λ results in a much smaller beam divergence angle θ∥ along the X axis, which is on the order of 10° for each individual beamlet. This means that optics with lower numerical aperture (slower optics) can be used for optical conditioning of the beamlets along the X axis. As a consequence, the X axis in FIG. 1 is generally referred to as the slow axis of a laser diode bar.
The structural characteristics of laser diode bars along with the quite different beam divergence angles along the fast and slow axes make any optical transformation of the plurality of raw laser beamlets a real challenge, particularly when the laser illumination output beam must meet stringent requirements. This challenge is better appreciated by pointing out the strong asymmetry between the spatial quality of the laser beamlets along both axes. The spatial quality of a laser beam can be conveniently quantified by the so-called beam parameter product (BPP). This parameter is given by the product of the minimum beam size (beam waist) measured along a given transverse direction with the beam divergence angle measured along the same direction. For example, the plurality of beamlets (taken as a whole) radiated by a typical laser diode bar has a BPPF of about 0.001 mm×1400 mrad=1.4 mm-mrad along the fast axis, while its counterpart BPPS along the slow axis reaches about 10 mm×175 mrad=1750 mm-mrad. Despite of the sizable beam divergence angle along the Y axis, the low BPPF along this axis indicates that the laser beamlets have a very high quality, which approaches that of an ideal Gaussian beam. Stated otherwise, the laser beamlets are nearly diffraction-limited along the fast axis. In turn, the spatial quality of the whole set of laser beamlets along the slow axis degrades dramatically according to the BPPS value that gets higher by nearly three orders of magnitude. The degraded beam quality along the slow axis comes, in one hand, from the laser emission that is formed of a set of individual laser beamlets that escape from emission apertures well spaced from each other and spread along a segment of 10-mm long. On the other hand, the intrinsic spatial quality of each individual beamlet along the slow axis is much lower than along the fast axis. This is due to the relatively large width W of each laser diode emitter (50 μm to 200 μm, as noted earlier), which favors laser oscillation on several higher-order lateral modes having intricate transverse irradiance profiles.
Note that the product of the BPP values along both fast and slow axes gives an optical beam metric having the units of area×solid angle, and it shares several features with the concept of étendue commonly used in the design of optical illumination systems.
The optical conditioning of a plurality of highly-divergent laser beamlets having asymmetric cross-sectional areas along the orthogonal fast and slow axes has been tackled with various approaches, one of them being illustrated in the schematic perspective view of FIG. 2 (PRIOR ART). Note that the expression “optical beam conditioning” is defined herewith as the transformation of the spatial characteristics of a laser beam (or a set of laser beamlets) using appropriate optics in order to fulfill specific requirements. For instance, this expression encompasses the collimation of a laser beam, which is an optical transformation that specifically aims at minimizing the divergence angle of the beam by increasing its minimum transverse beam size. FIG. 2 shows a laser diode system formed of a laser diode bar 10 comprising five emitters (depicted by the filled areas) and of beam conditioning optics 12. This optics includes a cylindrical microlens 14 placed close to the front facet of the laser diode bar 10 and dedicated micro-optics 16. The use of a single cylindrical microlens 14 to collimate the whole set of beamlets along the fast axis is well known in the art and disclosed in U.S. Pat. Nos. 4,785,459 to Baer (step-index optical fiber), 5,081,639 to Snyder et al., (aspheric microlens) and 5,825,803 to Labranche et al., (graded-index (GRIN) microlens). This technique can reduce the beam divergence angle to less than 1° along the fast axis while achieving optical throughputs of about 90%, so that less than 10% of the incident optical power is lost through the fast-axis collimation step.
The function of the micro-optics 16 can be imagined as a rotation by 90° of the fast-axis and slow-axis divergence angles of each beamlet that impinges on its input facet. Various designs for the micro-optics 16 have been disclosed in prior art patents, such as in U.S. Pat. Nos. 5,168,401 to Endriz, 5,513,201 to Yamaguchi et al., 6,324,190 to Du et al., and in 6,870,682 to Grenier et al., all of these inventions relating to reflective micro-optics. In turn, designs based on refractive micro-optics are disclosed in U.S. Pat. Nos. 5,513,201 to Yamaguchi et al., 6,639,727 to Kusuyama, and in 6,471,372 to Lissotschenko et al. The effect of the rotation of the individual beamlets by 90° about their propagation axis Z is depicted in FIG. 2 by the sets of hatched rectangular areas enclosed in two separate boxes, one of them 18 pertaining to the beamlets incident on the micro-optics 16 while the other 20 pertains to those leaving it. The micro-optics 16 is often referred to as a beam symmetrization device because its action results in a set of output beamlets that have their beam parameter products BPPF and BPPS closer to each other, as compared to the original pair of BPP factors, prior to optical beam conditioning. Beam symmetrization devices are available with optical throughputs that can reach 90% when used with laser diode bars with fill factors of about 20% to 50%. In addition, some beam symmetrization devices are packaged with a fast-axis collimation microlens 14, so that the optical conditioning of the whole set of laser beamlets can be realized with a single component.
The beam symmetrization devices disclosed in the various patents cited in the preceding paragraph are not well suited for use with laser diode bars having high fill factors because the incoming beamlets must be sufficiently spaced from each other to minimize truncation of their irradiance distributions upon entering into the device. A refractive-type beam symmetrization device that reduces truncation of the incoming beamlets is disclosed in U.S. Pat. No. 7,260,131 to Grenier et al. Both input and output surfaces of this device have curved profiles and properly-shaped contours that better match the irradiance distributions of the incoming beamlets. The device allows for emission of a set of conditioned beamlets with increased radiance (or brightness) since the gap between consecutive beamlets in the output plane of the device is minimized, if not totally eliminated. Accordingly, a beam symmetrization device that allows sizable reduction of the full cross-sectional area of the beamlets is disclosed in U.S. Pat. No. 7,027,228 to Mikhailov. The narrower beam irradiance distribution at the output of the device promotes better focusing of the beam power on the input face of an optical fiber.
Some highly-demanding illumination tasks call for optical output powers that largely exceed what a laser illuminator a made up of a single laser diode bar can deliver when driven at its maximum current rating. A way to promote higher output powers consists in integrating a plurality of laser diode bars in the laser illuminator assembly and to couple optically their individual laser emissions. The quest for powerful laser illuminator devices then lends itself to the use of stacked laser diode arrays (SLDAs), which are built by stacking several identical laser diode bars in a monolithic package. By way of example, FIG. 3 (PRIOR ART) shows a schematic front view of a SLDA made up of five laser diode bars depicted by the gray-shaded areas. Each laser diode bar comprises ten emitters, having their front facets depicted by the rectangular areas filled in black. The laser diode bars can be stacked one above the other according to a suitable packaging architecture to give a two-dimensional laser diode array, which would comprise a total of 50 laser emitters for the specific example depicted in FIG. 3. The packaging architecture serves to hold the laser bars firmly in place while ensuring proper electrical biasing and cooling of each bar. The radiated total output power scales directly with the number of stacked laser diode bars. Some packaging architectures allow the vertical spacing P between consecutive laser diode bars, denoted as the array pitch, to be as low as 0.4 mm.
Two packaging architectures are widely used for mounting laser diode bars in SLDAs. The first architecture encompasses the numerous variations of the Rack-and-Stack technique, in which the laser diode bars are first mounted on individual submounts or heatsink assemblies that are then stacked. SLDAs packaged according to this architecture are disclosed in U.S. Pat. Nos. 4,716,568 to Scifres et al., 4,719,631 to Conaway, 5,099,488 to Ahrabi et al., 5,305,344 to Patel, 5,311,530 to Wagner et al., 5,715,264 to Patel et al. and in 5,764,675 to Juhala. The other packaging architecture is based on the Bars-in-Grooves technique, in which the laser diode bars are inserted into parallel grooves machined with high precision in a substrate made of a material having high thermal conductance. The bars are soldered to the side walls of the grooves, thus resulting in very rugged, monolithic assemblies. This packaging architecture is disclosed in U.S. Pat. Nos. 5,040,187, 5,128,951, 5,284,790, and 5,311,535, all to Karpinski.
The beam conditioning optics discussed in the preceding paragraphs serves for optical transformation of the plurality of beamlets radiated by a single laser diode bar. As a consequence, they cannot be used as is for conditioning the laser emission from a SLDA, which takes the form of a two-dimensional array of beamlets having different spacings along the orthogonal fast and slow axes. Obviously, one may think of stacking several beam conditioning components along the vertical direction in nearly the same way as for the diode laser bars, and then placing the whole assembly in front of the SLDA to perform a collective transformation of the beamlets.
Unfortunately, this approach often leads to disappointing results, due to factors such as mismatches between the vertical pitches of the SLDA and of the beam conditioning stack, the overall height of the beam conditioning components that exceeds the array pitch, variations of the array pitch along the whole stack, and possible vertical misalignment of the laser emitters belonging to the same vertical column in the stack. Stated otherwise, the optical registration of the beam conditioning components with the individual laser emitters in the SLDA calls for tight manufacturing and mounting tolerances. The short focal lengths required for conditioning beamlets having strong divergence angles along both fast and slow axes amplify the detrimental effects of the factors mentioned above. These factors act together in degrading the overall throughput of the beam conditioning stack while giving a final output laser beam with spatial characteristics that, most of the time, do not fulfill the stringent requirements of several laser illumination applications.